This invention relates to a relatively simple efficient and economical process for removing particulates and gases such as sulfur oxides, hydrogen sulfide and organic sulfur compounds from an industrial gas stream. Mixed emissions of this type are commonly found, for example, in Kraft and sulfite recovery processes in the pulp and paper industries. Prior art processes teach various methods of removing these types of emissions individually, however, none of the prior art teaches an economical coordinated process for the removal of all of these components. Furthermore, in some cases, a prior art process for the removal of one component interferes with or reduces the efficiency of subsequent removal steps for other components.
For example, emissions from Kraft recovery boilers typically consist of hydrogen sulfide and organic sulfur compounds (designated "TRS" for total reduced sulfur), SO.sub.2 and particulates. The organic sulfur compounds typically consist of mercaptans such as methyl mercaptan (CH.sub.3 SH), mercapto ethers such as dimethyl sulfide (CH.sub.3 SCH.sub.3), and disulfides such as dimethyl disulfide (CH.sub.3 S.sup.. SCH.sub.3). Some references indicate the presence of carbonyl sulfide (COS). The quantity and composition of emissions are a function of boiler feed and loading, boiler operation, and process sulfidity.
Emissions from boilers are generally in the broad range of:
Trs: 10-2500 ppm (parts per million) PA0 Particulates: 1-7 gr/sdcf (grains per standard dry cubic foot) PA0 So.sub.2 : 10-200 ppm (parts per million).
The permissible emissions from recovery boilers are, increasingly, being restricted by govenment authorities. Although the level of restriction varies with the specific authority, the emerging standards for 1977 appear to be TRS less than 5 PPM and particulates less than 0.08 to 0.04 gr/sdcf.
In some new boiler designs, TRS emissions can be controlled to 3-10 PPM when operating at 80-100% of design capacity, but only with close combustion control and decreased thermal efficiency. Also, particulate emissions present more of a problem with this type of design. Black liquor oxidation processes in combination with existing furnaces can, with close control, maintain TRS emissions at 4-30 PPM when operating at 80-100% of design capacity, but the particulate emissions problem still exists. Electrostatic precipitators in existing recovery boilers, after an extended period of operation such as 3-5 years, are reducing particulate emissions to levels of 0.10-0.25 gr/sdcf at 80-100% of design capacity. When the boilers are operated at 120% of design capacity, however, the particulate emissions level in many cases increases to more than 1 gr/sdcf. None of these systems can readily accommodate fluctuating boiler load levels. Furthermore, electrostatic precipitators in themselves do not control TRS emissions. Therefore, it appears that neither electrostatic precipitators alone, black liquor oxidation alone, nor a combination of these two well-evaluated systems, are consistently capable of meeting the overall environmental regulations.
Recently, experimental work has been conducted on the absorption of sulfur oxides and other sulfur compounds in alkaline slurries of activated carbon. In particular, U.S. Pat. Nos. 3,701,824; 2,823,766; 3,486,852; and 3,824,163 teach that water slurries of activated carbon can be used to scrub sulfur dioxide, hydrogen sulfide, and organic sulfur compounds such as mercaptans and alkyl sulfides from a gas stream. These patents appear to depend on a combination of sorption and oxidation processes. In general, these patents teach a carbon slurry concentration of about 0.1-10% by weight or higher for the cocurrent or countercurrent scrubbing of sulfurous gases having hydrogen sulfide or organic sulfur compound concentrations on the order of 100-5000 PPM. These patents do not discuss the problem of the removal of particulates.
Other prior art patents disclosing alkaline scrubbing reactions are U.S. Pat. Nos. 3,852,408; 3,852,409 and 3,755,990.
U.S. Pat. No. 3,324,630 teaches a process for removal of particulates from a gas stream which utilizes a crossflow scrubbing technique, and the disclosure of this patent is incorporated herein by reference. The process disclosed is capable of removing very small particulates on the order of 0.1-10 microns in size.
In the aforementioned U.S. Patent Application Ser. No. 463,652, an improvement in scrubbing is disclosed wherein the particulate-laden gas stream is first treated under substantially adiabatic conditions to increase its turbulence and to increase its humidity substantially to saturation at a temperature above about 150.degree. F. to initiate nucleation of small particulates by condensation and/or agglomeration. Thereafter the gas is contacted with a scrubbing liquor which can be recirculated through a packed enclosure, usually at a substantially constant temperature. This improvement normally eliminates the need for cooling the recirculating liquor at a saving in material and energy costs.